1. Field of the Invention
This invention relates to the preparation of self-crosslinking copolymers derived from an N-hydroxyimide, such as N-hydroxysuccinimide blocked isopropenyl- alpha, alpha-dimethylbenzyl isocyanate, and a hydroxy group containing olefinically unsaturated monomer. The self-crosslinking copolymers are useful as self-curing resins in coatings, adhesives, and moldings.
2. Description of the Related Art
In general, commonly used urethane coatings systems are of the two-component or one-component types.
A typical two-component paint, for example, comprises a polyisocyanate material and a polyhydroxy material, and, upon curing, can form films with excellent physical properties and resistance properties. They are used in high performance automotive coatings.
A typical one-component paint, for example, comprises a blocked polyisocyanate and a polyhydroxy material. Because the isocyanate groups are blocked, one-component systems have a generally longer shelf life than two-component systems containing free, hydroxy-reactive isocyanate groups. When desired, the one-component systems may be activated by, for example, heat, to deblock the blocked polyisocyanate and promote crosslinking.
Japanese Patent No. 63-186,722 discloses a coatings system wherein the blocked isocyanate and hydroxy groups are part of the same polymer backbone, thereby producing a self-crosslinking type one-component system. Similar ethanol-blocked copolymers are disclosed in JP 1-087,606.
U.S. Pat. No. 5,116,930 discloses another approach to a self-crosslinking resin accomplished by hydroxyfunctionalization of a polyisocyanate polymer. It is stated therein in column 1 and in Comparative Example 3, column 9, that when a self-curing resin is produced by copolymerizing an oxime-blocked isocyanate-containing radical-polymerizable monomer and a hydroxyl containing radical-polymerizable monomer, the blocking agent may undergo dissociation during polymerization disadvantageously leading to gelation, and, at the same time, leading to marked polymer discoloration. Furthermore, we have ourselves observed on numerous occasions the setting up of the contents of the reactor into a crosslinked gel during attempts to prepare the above-mentioned copolymers.
A solution for the gelation problem has been described in the co-pending application submitted concurrently herewith entitled "Process for the Preparation of Gel-Free Self-Crosslinking Copolymers Derived from Blocked Isopropenyl-Alpha, Alpha-Dimethylbenzyl Isocyanate," U.S. patent application Ser. No. 07/963,880, filed Oct. 20, 1992.
Another solution of the gelation problem is described herein. The solution comprises using a novel N-hydroxyimide-blocked isopropenyl-alpha, alpha-dimethylbenzyl isocyanate which:
1. has a higher deblocking temperature than oxime- or lactam-blocked isopropenyl-alpha, alpha-dimethylbenzyl isocyanates so that premature deblocking and gelation is prevented, but PA0 2. has a lower deblocking temperature than alcohol-blocked isopropenyl-alpha, alpha-dimethylbenzyl isocyanates so that rapid cure is still obtained at conveniently low cure temperatures. PA0 alkyl nitriles PA0 cyclic sulfones PA0 N-alkyl pyrrolidones PA0 N,N-dialkyl formamides PA0 N,N-dialkyl acetamides PA0 alkyl formates PA0 alkyl oxalates PA0 hexamethylphosphorus triamide (HMPT) PA0 hexamethylphosphoramide (HMPA)